The invention relates to methods of partially applying a hot-metal pressure sensitive adhesive composition to a backing material.
Substrates coated with highly viscous materials are known in the medical sector. From certain standpoints it is sensible for these coatings not to have impervious surfaces but instead to be applied in the form of dots, which allows, for example, the skin under bandages not to macerate, owing to the possibility for perspiration and other skin excretions to depart. An appropriate method for achieving this dot-form coating is that of rotary screen printing.
In this method, there is a rotating screen within which is located a nozzle, by means of which the fluid to be applied is introduced from the outside into the screen chamber and pressed through the screen perforations in the direction of the substrate to be coated. The screen is lifted from the substrate in accordance with the rate of transportation of the substrate (rotary speed of the screen drum). As a consequence of the adhesion and the internal cohesion of the fluid, the limited supply of hot-melt adhesive composition in the perforations is drawn in sharp definition from the base of the domes that is already adhering to the backing, and is conveyed onto the backing by the prevailing pressure.
After the end of this transportation, the more or less strongly curved surface of the dome forms over the specified base area, in dependence on the rheology of the fluid. The height-to-base ratio of the dome depends on the ratio of the perforation diameter to the wall thickness of the screen drum and on the physical properties (rheology, surface tension and contact angle on the backing material) of the fluid.
Numerous substrate materials based on films, wovens, knits, nonwovens, gels or foams have already been disclosed and are also employed in practice. In the medical sector there are particular requirements imposed on the backing materials. The materials are required to be skin compatible, generally permeable to air and/or water vapor, and also readily modelable and conforming. On the basis of these requirements, a very thin or soft backing is frequently preferred. For handling and in use, however, the backing materials are also required to be of sufficient strength and possibly of limited extensibility. Furthermore, the backing material should retain sufficient strength and low extensibility even after becoming wet through.
In the textile industry it is also known that partial coatings may be transferred. EP 0 675 183 A1 describes a process which transfers hot-melt adhesive geometries onto a specially crosslinked substrate. An intermediate backing which has been rendered dehesive is likewise mentioned in EP 0 356 777 A1. The use of a coated counterpressure roll as auxiliary backing for the transfer is also described (CH 648 497 A5), where self-adhesive products are not addressed.
The design of nozzle and screen is described fundamentally in CH 648 497 A5; improvements of the process are described in EP 0 288 541 A1, EP 0 565 133 A1, EP 0 384 278 A1, and DE 42 31 743 A1.
It is known, furthermore, from EP 0 622 127 B1 that pressure sensitive, solvent free adhesive films are applied to a substrate via a roller. Single channel or multichannel nozzles are employed as applicator units.
Owing to an applied difference in speed between the coated roller or the decreasing substrate, the metered film of adhesive is reduced in its thickness, so that thin films of pressure sensitive adhesive may be transferred to substrates. However, value is always placed on the fact that the thin films of pressure sensitive adhesive are impervious, without voids, in order to ensure uniform quality of the product.
Another common feature, especially with coatings of medical backing materials, is to provide the backing with an impervious film of composition which is subsequently perforatedxe2x80x94in an additional process stepxe2x80x94by means of compressed air, spike rollers or lasers.
It is an object of the invention to provide a method that is outstandingly suited to apply a hot-melt pressure sensitive adhesive to a backing material in such a way that the backing material thus coated, when employed as a plaster, has improved properties in respect of breathing activity or water vapor permeability.
This object is achieved by a method as described in the main claim. The subsidiary claims relate to advantageous developments of the subject matter of the invention.
The invention accordingly describes a method of partially applying a hot-melt pressure sensitive adhesive composition to a backing material, in which
the hot-melt pressure sensitive adhesive composition is applied by an applicator to a moving transfer means in such a way that a film formed from the hot-melt adhesive composition comprises voids;
the applied film is subsequently applied to a backing material, which is likewise moving.